FAQ - Answer
The problem may be related to summer irrigation of the oaks, which is highly conducive to oak root fungus infections and water mold root rot infections (Phytophthora spp). Many oaks decline as the result of summer irrigation - blue oak included. If the problem is related to summer irrigation, then it would be good to change the understory from irrigated landscape to mulch (wood chips, bark, or natural leaf and twig litter) or plants compatible with oaks. We identify such plants in chapter 5 of Oaks in the Urban Landscape (UC publication 3518) and give a fair amount of info regarding appropriate cultural practices for oaks. The key element to change is the irrigation - maintaining relatively dry conditions around the base of the trunk in the summer months is very important for CA native oaks.